1. Field of the Invention
The present invention relates to a wireless communication technique, in particularly, to a signal separation device and a signal separation method for separating plural signals, transmitted from transmission devices and received by a receiver, into individual transmission signals.
2. Description of the Related Art
In the wireless communication technical field, studies are being made to realize large capacity and high speed communication of the present generation and the next generation communication systems. For example, in addition to an existing SISO (Single Input Single Output) scheme, from the point of view of increasing the communication capacity, studies are being made on a SIMO (Single Input Multi Output) scheme, a MISO (Multi Input Single Output) scheme, furthermore, a MIMO (Multi Input Multi Output) scheme.
FIG. 1 is a diagram illustrating a communication system adopting the MIMO scheme, which includes a transmitter 102 and a receiver 104.
As illustrated in FIG. 1, in the MIMO scheme, signals from a number of transmitting antennae 106-1 through 106-N are transmitted simultaneously at the same carrier frequency. These signals are received by a number of reception antennae 108-1 through 108-N. Here, it is assumed that the number of the transmitting antennae is the same as the number of the reception antennae just for simplicity of description; certainly, the number of the transmitting antennae can be different from the number of the reception antennae.
In the receiver 104, the signals collectively received by the reception antennae 108-1 through 108-N are separated into those individuals signals transmitted from the transmitting antennae 106-1 through 106-N. The separated signals are sent to later-stage elements for demodulation.
There are several methods for the receiver 104 to separate the received signals. One is the so-called MLD (Maximum Likelihood Detection) method. In MLD, a squared Euclidian distance is calculated throughout all possible combinations of the signals transmitted from the plural transmitting antennae and the received signals, and the combination resulting in the minimum distance is selected.
By using MLD, although the collectively received signals can be reliably separated into individual transmitted signals, because deduction of the squared Euclidian distance requires a large number of calculations, signal separation with MLD requires high calculation capability.
For example, if four signals (x1, x2, x3, and x4) are transmitted from four transmitting antennae by using the 16 QAM modulation scheme, in this case, each of the four transmission signals is mapped into one of 16 signal points in a signal constellation diagram (a diagram illustrating distribution of signal points), the number (M) of all possible combinations of the transmission signals in the received is expressed as PN, where, P indicates the number of signal points of one transmission signal, and N indicates the number the transmitting antennae. In this example, as mentioned above, P=16, and N=4, hence, M=164=65536, that is, there are as many as 65536 different combinations, and it is a very large number.
In order to calculate the squared Euclidian distance for all these combinations to determine a most-probable combination, very high calculation capability is required, and this makes it difficult to reduce the size of a mobile terminal. Further, because of the large number of calculations, electrical power consumption increases, and this also hinders reduction of the size of the mobile terminal.
Another method of separating the received signals is the so-called QRM-MLD, which involves corrections to MLD. In QRM-MLD, QR separation and M-algorithm are applied to MLD so as to decrease the number of computations required in calculation of the squared Euclidian distance.
For details of QRM-MLD, for example, reference can be made to “K. J. Kim, et al., “Joint channel estimation and data detection algorithms for MIMO-OFDM systems”, Proceedings 36th Asilomar Conference on Signals, Systems and Computers, November 2002”.
Considering the above example again, when using QRM-MLD, the number (MC) of computations in calculation of the squared Euclidian distance is expressed as:MC=NA+NB*NC*ND,
Where, NA represents the number of signal point candidates, NB represents the number of newly added signal point candidates, NC represents the number of surviving signal point candidates at the previous stage, and ND represents the number of the transmitting antennae.
As NA=16, NB=16, NC=16, and ND=3, it is obtained that MC=16+16*16*3=748.
Therefore, by using QRM-MLD, the large number of the calculations can be reduced greatly compared to MLD. However, considering a compact mobile terminal, this number of calculations is still too large.